The fuel cell is a power generation device that derives electric energy by electrochemically oxidizing fuel such as hydrogen and methanol, and in recent years, it has attracted attention as a clean energy source. Among others, the polymer electrolyte fuel cell, which normally works at a low operating temperature around 100° C. and has a high energy density, is expected to serve in a wide variety of fields as relatively small distributed power generation facilities and power generation equipment for movable bodies such as automobiles and ships. Furthermore, it has attracted attention as a power source for small movable devices and portable appliances and, in particular, it is expected to replace secondary batteries such as nickel hydrogen batteries and lithium ion batteries incorporated in portable telephones and personal computers.
A fuel cell commonly includes cells as units, each composed mainly of a membrane-electrode assembly (hereinafter occasionally abbreviated as MEA) sandwiched between separators. A MEA consists mainly of electrodes, i.e., an anode and a cathode where power generating reaction takes place, and a polymer electrolyte membrane that works to conduct protons between the anode and the cathode. The main component of the polymer electrolyte membrane is an ionic group-containing polymer (polymer electrolyte material). To increase durability, a polymer electrolyte composition compounded with an additive and the like may also be used. The polymer electrolyte composition is also suitable as a binder or the like in an electrode catalyst layer for use in a particularly severely oxidizing atmosphere. As for the required characteristics of the polymer electrolyte membrane and the polymer electrolyte composition, high proton conductivity is first cited and, in particular, they should be able to maintain high proton conductivity even under high temperature, low-humidified conditions. Furthermore, polymer electrolyte membranes and polymer electrolyte compositions are required to be low in permeability to fuels to function as a barrier to prevent direct reaction between fuels and oxygen. Other required characteristics include chemical stability to resist an oxidizing atmosphere during fuel cell operation, as well as mechanical strength and physical durability to resist thin film formation and repeated swelling-drying cycles.
So far, as the polymer electrolyte membrane, NAFION (registered trademark) (manufactured by DuPont), which is a perfluorosulfonic acid based polymer, has been widely used. NAFION (registered trademark), which is manufactured through a multistep synthesis process, is very expensive and has an issue that fuel crossover is great. Furthermore, a problem of being low in softening point and unable to be used at high temperature, a problem of after-use disposal process, a problem of materials thereof being difficult to recycle, and so on have been pointed out. Furthermore, as a polymer electrolyte membrane low in cost and excellent in membrane characteristics which can replace NAFION (registered trademark), hydrocarbon based electrolyte membranes have in recent years been being developed more and more actively.
However, these polymer electrolyte membranes all have a problem of the chemical stability falling short when used in a polymer electrolyte fuel cell. The mechanism of the chemical degradation has not been sufficiently elucidated. However, it is conceivable that hydrogen peroxide generated mainly at the electrode during electricity generation or hydroxy radicals generated by the aforementioned hydrogen peroxide reacting with iron ions or copper ions present in the membrane cuts polymer chains or side chains so that the polymer electrolyte membrane has a reduced membrane thickness or becomes weak. Moreover, there is a problem that, as swell and shrinkage occur repeatedly with changes in humidity, the weakened polymer electrolyte membrane breaks resulting in failure of electricity generation.
In the above situation, there have been conducting studies to improve the mechanical strength and chemical stability and improve the durability by using a polymer electrolyte composition applying perfluoro-based electrolyte membrane and hydrocarbon-based electrolyte membrane each containing antioxidant.
For example, International Publication WO 2008/102851 proposes a polymer electrolyte composition in which a perfluorosulfonic acid based polymer has been compounded with a polyphenylene sulfide (hereinafter, sometimes referred to simply as PPS), which is a sulfur-containing polymer, and a polybenzimidazole (hereinafter, sometimes referred to simply as PBI), which is a nitrogen-containing polymer.
Japanese Unexamined Patent Publication (Kokai) No. 2005-350658 proposes a polymer electrolyte composition in which a perfluorosulfonic acid based polymer or a sulfonic acid group-containing polyether ketone based polymer (hereinafter, sometimes referred to simply as sPEK) is compounded with polyamic acid or polyimide.
Japanese Unexamined Patent Publication (Kokai) No. 2013-80701 proposes a polymer electrolyte composition in which a perfluorosulfonic acid based polymer or sPEK is compounded with insoluble PBI particles.
The composition proposed in International Publication WO 2008/102851, however, is still poor in long-term durability.
The composition proposed in Japanese Unexamined Patent Publication (Kokai) No. 2005-350658 is poor in both electricity generating ability and long-term durability although the permeability to methanol is low as a result of heat treatment after adding polyamic acid.
The polymer electrolyte composition proposed in Japanese Unexamined Patent Publication (Kokai) No. 2013-80701 is still poor in long-term durability although the durability of the composition is somewhat improved by adding PBI to decompose the hydrogen peroxide and hydroxyl radicals.
Thus, polymer electrolyte composition manufactured by conventional techniques cannot work sufficiently to improve economic efficiency, processability, proton conductivity, mechanical strength, chemical stability, and physical durability and therefore cannot serve as a useful polymer electrolyte composition for industrial applications.